The present invention relates to a compensator for liquid crystal display and more particularly to a compensator for an active matrix type liquid crystal display (hereinafter referred to simply as "TFT-LCD") using a thin film transistor.
Liquid crystal display occupies an important position in the display field because of such characteristic features as being driven at a low voltage, light weight and low cost. Above all, a TFT-LCD is high in response speed, relatively small in viewing angle dependence and affords a clear color image, and because of these characteristics the TFT-LCD is most expected as the next generation LCD. Researches and developments are being made actively. In the case of a TFT-LCD, however, it is very difficult to manufacture an LCD having a screen size of 10 inches or so in diagonal angle, and in addition to this serious problem, the performance thereof is not completely satisfactory yet. If the cell gap is thinned for the purpose of improving performance, for example, improving the response speed, improving the viewing angle characteristic, or decreasing the driving voltage, the product .DELTA.n.multidot.d of the birefringence of liquid crystal, .DELTA.n, and the cell gap, d, becomes smaller and the mode of light travelling through the liquid crystal is no longer a linear polarization but becomes an elliptical polarization. As a result, there arises the problem of interference color when combined with a polarizer This phenomenon is more marked as the value of .DELTA.n.multidot.d called Morgan limit becomes smaller than Morgan limit (about 2 .mu.m), leading to deterioration of the display quality such as contrast and color quality.
The interference color created by such decrease of the cell gap can be cancelled by disposing on a display cell a compensator capable of making a transmitted light as an eliptically polarized light after passing through the display cell again into a linearly polarized light. This compensating method has already been used practically in simple matrix LCD's of a super-twisted nematic (STN) type. More concretely, there are known a double-cell compensating method further using another liquid crystal cell for compensation, and a retardation film compensating method. However, the former method makes further difficult the manufacture of TFT-LCD which is originally difficult, because of heavy weight and large thickness, so is never preferable. On the other hand, the latter method is insufficient in its ability of cancelling the interference color and so is not satisfactory as a compensation method for TFT-LCD which is greatly characterized by a high display quality.
It is the object of the present invention to solve such problems of the prior art.